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Can someone identify this biplane from 1920-21 taken in Arkansas?
Can someone identify this biplane? It was taken in late 1920 or early 1921 when my grandfather was barnstorming in Arkansas. My grandmother and aunts are also in the photos. Additionally, can someone identify where in Arkansas this Hickman Air_____ was located?

I have no way to confirm if it was the same place, but there is a private airfield called “Hickman Airport” near Black Oak, AR.  I looked around on google maps and couldn’t actually fins anything resembling a usable grass runway any more.  The AirNav page identifies at as being at 35.8311056,-90.3704139.  See also:  http://www.airnav.com/airport/2AR8

Thanks for the info

"Hickman Air..." looks like it's followed by a "B", not an "F" or "P". However, I can find no record of a "Hickman Airbase", the closest an internet search comes up with is "Hickam Air Base" in Oahu. I smidge too far away to be a likely candidate...

Thanks for the response

It looks like a Curtiss Oriole. Note the smooth and rounded fuselage and how it comes to a rounded point under the tail. Additionally, note the engine manifold covers and some sort of extended "stack" from the rear of the engine.

Image source: Northwest Airlines History Center


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Could we make a Hybrid Plane, But not use batteries?
What if we had wind turbines on planes. These would not power a battery but would instead power a smaller engine. Perhaps once a pilot goes to the cruising level he could activate the wind turbines to come out from the bottom of the wing, or some other more sensible spot. These would spin very fast and in turn, the motion would be converted to electricity. Although this would not be enough to keep the plane flying it would help the plane conserve fuel, similar to hybrid cars.
This would already be in use if it worked, or it would be on the news, so why does it not work? And if it works why not use it?

Does this answer your question?

Century old idea....

@user3528438 Much older than that.  I'm pretty sure da Vinci either designed a PMM, or debunked one.  I think one of the Greek philosophers (Archimedes?) had one sketched out.

Just think about what you are asking:  You want to put a propeller in the airstream to turn a generator, which will power a motor, which will turn a propeller.  Even without an engineering degree you should understand that there are energy losses due to inefficiency in the system and the absolute best you could hope for is to break even.  So if you can only break even, why bother?  Just fly a glider!  It's almost like a photo I saw of someone who plugged a power strip into itself and didn't know why it wouldn't work.

Ram Air Turbines (RAT)  are on many aircraft:  https://en.wikipedia.org/wiki/Ram_air_turbine  They add a lot of drag, so are not there for any propulsion purpose, as are suggesting.

Much like the wheeled vehicle with propeller or windmill geared to wheels, and no motor, which can supposedly go downwind faster than the wind.  (Kidding!  That one's a real thing.)

A different riff on your idea would be an electric motorglider that has the option to use the propeller as a generator to charge batteries as one way of storing energy available from updrafts, as opposed to the usual method of simply gaining altitude until the top of the updraft is reached. I was thinking about this idea way back when I was a little kid. This may have actually been done in some of the recent long-range solar-powered airplane flights but maybe not; I think some of these aircraft were so lightly constructed that they tried to stay away from turbulent updrafts.

You people (not talking to the OP) on this site need to stop being such c*nts and downvoting these questions.  It’s an honest question and this kind of mobbing and bullying gets really tiring.

This is a perpetual motion scheme, and no that would not work as it does violate the second law of thermodynamics.
Said wind turbine scheme like that could generate electricity, but would add additional drag on to the airframe which would have to be compensated for by additional thrust (energy is conserved).  In a perfect world the total drag force on those wind turbines could be totally converted into thrust by an electric engine driven by them. However, as they say, there’s no free cup of coffee in this universe. There will be thermodynamic and entropic losses associated with such a design.  You could never break even and, even if you could, the additional weight of the systems would have to be accommodated my additional structure, weighing more, creating more drag and requiring even more thrust to overcome.
When I was a child, I had an idea for an airplane which was electrically powered.  An electric motor not only drove a propeller but also an electric generator which, in turn powered the motor.  My father, an engineer himself, quickly pointed out to me the fallacy of such designs.  Until we figure out a means to spontaneously create energy - and that’s not necessarily out of the question if you subscribe to the multiverse and string theory physics! - they remain the stuff of childhood fantasies.

EDIT: Beyond the tongue in cheek vid above, here’s a pretty good one on perpetual motion.

Current science does not support the concept of perpetual motion.
You can not extract energy out of nowhere.
Sailing ships, including wind turbine powered ones, can sail against the wind, but they do so by extracting energy from the interface between wind and water - transferring wind's energy to water and using some of it for propulsion.
There is no such interface for an airplane, unless it's sitting on the ground. In theory one could get energy from an interface between thermal currents, but that is better done without a turbine, by a glider.

The simple reason this isn't in use is it won't and can't work.
Under no circumstances can you obtain even as much energy from your array of Ram Air Turbines as they will sap from the aircraft's motion -- it'll be like trying to save fuel by deploying a drag brake.
Why?  Because of what gets lumped together as "losses."  A windmill turbine is less than 100% efficient in turning wind movement into shaft power, the generator behind it is less than 100% efficient at converting shaft rotation into electrical energy, your electric flight motor (to use this energy) is less than 100% efficient at turning electrical energy back into shaft rotation, and its propeller is less than 100% efficient at turning shaft rotation into forward thrust in the form of airstream kinetic energy.
The result is that you get less thrust from the drive motor than the drag produced by yoru array of wind turbines, and the whole arrangement winds up being worse than just shutting off the engine and gliding.
If you want to get the most distance for the least energy cost, look into self-launching sailplanes.  They use fuel or electricity (stored in relatively small batteries) to get from the ground to soaring altitude, then not only shut down, but stow the motor and propeller, and use energy harvested from the atmosphere to fly at cross-country speeds comparable to a lower end light airplane (competition sailplanes can fly point-to-point at a net speed of up to 100 knots in suitable weather)

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Is anyone able to identify this vintage aircraft please?
I have this plane on an art glass print in my home.  A lot of people ask what the plane is and I have been unable to identify it.  Any ideas?

It appears to be a Spartan 7W Executive.

I think the risk of Wikipedia vanishing is pretty low risk. What would be useful here would be a list of the key features (eg. two blade prop, gear configuration, cabin structure, exhaust ports, etc) that might have informed this conclusion.

Yes, it is unmistakingly a Spartan Executive.

As posted above, it is a Spartan Executive - the ultimate luxury airplane for the discerning private aviator in the late 1930s. They were a good size and very fast - the Gulfstream of their day, and as aspirational as a Lamborghini is to today's kids.
Quite a few are still flying, and I have seen several.
Here is a decent air-air photo of one, showing off its lines.

It is considered good form to cite the source of posted photos.  Would you please add a link?

No original link available, sorry, it came from a recovered hard drive stripped of info, and is probably old enough to predate the internet archive.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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What aircraft are these, and why are these engine sizes so different?
These glamorous vintage images were once famous in magazines and used by airlines to advertise their products, and yes, flight attendants clothing was like this! Old and more relaxed times, though flying was for the rich.

Left: Canadian Pacific, 1968.
Right: Pacific Southwest -  San Diego Air and Space Museum Archive, likely end of 60s too.
With a flight attendant showing the scale, it's pretty clear that the size of the engines are completely different; they are likely of different designs (turbojet/turbofan) in spite of being of the same period. What are theses engines, and the aircraft that used them? Why so different choices (what did that change for the airlines)?

Why so different? "Mines bigger than yours!"

One hopes they didn't try doing that with the engines running.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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How can I keep my A&P license active without being in aviation?
I am no longer in aviation. I do not want my A&P license to go dormant. Are there any ways to keep my A&P license active without being in aviation for over two years?

You might consider volunteering with an EAA or CAF chapter.  Your profile says you're from Wisconsin https://www.cafwi.org/

If you are "not in aviation", then I assume you are not working in/around/on airplanes. You are required to keep currency as per 61.83:

###§ 65.83 Recent experience requirements.

A certificated mechanic may not exercise the privileges of his certificate and rating unless, within the preceding 24 months -

(a) The Administrator has found that he is able to do that work; or
(b) He has, for at least 6 months -
(1) Served as a mechanic under his certificate and rating;
(2) Technically supervised other mechanics;
(3) Supervised, in an executive capacity, the maintenance or alteration of aircraft; or
(4) Been engaged in any combination of paragraph (b) (1), (2), or (3) of this section.

So the only way you can keep your currency (or regain it after you become non-current) without doing anything in Aviation is to demonstrate to the "Administrator" that you are able to do that work. What this entails is not defined in the CFR's, so will be up to your local FSDO.
Otherwise you can take a few hours a week and do mechanic work, for example you may be able to find a flight club and do some fleet maintenance (exchange it for instruction time or something).
tl;dr; You can't keep your license current without working on airplanes or supervising other mechanics. It would be analogous to keeping your pilots license current without ever flying.


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Where can I find no fly zones such as active stadiums while doing flight planning?
I am a student pilot in training for a PPL. I read that it is illegal to fly over a stadium filled with people, but not illegal to fly over stadiums that are not filled. Obvious reasons, of course.
When I'm doing flight training, where do I find this information of no-fly zones? On my TAC I see stadiums as visual reference points. How do I know if it is currently labeled as a no fly zone? Do I just look up the schedule for those stadiums on the stadium website or something? Or will it come up in the notams? I briefly remember seeing a live map where zones with no fly restrictions are visible, but I can't recall where.
Secondly, am I using the right terminology when I say no-fly zone? Or is there a more accurate term, similar to "hotspots" at airports?

Some cynics will say that the "obvious reasons" are not for security but instead so the fans don't see, e.g., unauthorized and un-properly-paid-for advertisements via banner tow... and see also the permanent "T"FR around Disneyland... see also this question.

The terminology you are looking for is Temporary Flight Restriction:

TEMPORARY FLIGHT RESTRICTION (TFR)- A TFR is a regulatory action issued by the FAA via the U.S. NOTAM System, under the authority of United States Code, Title 49. TFRs are issued within the sovereign airspace of the United States and its territories to restrict certain aircraft from operating within a defined area on a temporary basis to protect persons or property in the air or on the ground.

The "Stadium TFR" is a singular NOTAM, currently FDC NOTAM 0/0367, which creates a TFR with dimensions "3 NMR OF A QUALIFYING STADIUM OR OTHER SPORTING VENUE HOSTING A QUALIFYING EVENT UP TO AND INCLUDING 3000FT AGL ... ONE HOUR BEFORE THE SCHEDULED START UNTIL ONE HOUR AFTER THE END OF A QUALIFYING EVENT." It is entirely the pilot's responsibility to know which stadiums and sporting events are covered (there is a definition farther along the NOTAM) and what the exact start and end times are. As you say, stadiums should be marked on VFR charts as a red diamond, but the lack of such marking would (I am sure) not be an acceptable defense if you were found to have violated the TFR.
You can google the term "Stadium TFR" to find articles and forum posts discussing it. Note also that you are allowed to fly within the TFR if you meet certain specified requirements, such as if you have an operational reason to be there—as one example, many stadium TFRs encroach on the final approaches to Class B airports, and you can be sure that the airlines don't stop all their flights for those four hours.

Currently active TFRs can be viewed in graphical form at the FAA's website and at third-party sites like Skyvector.com. Skyvector will display active TFRs in red and upcoming TFRs in orange; I'm not sure about the FAA's site. I believe electronic flight bags like ForeFlight and Garmin Pilot also have this capability.
Note that the sites are probably all displaying the end times based on the scheduled end time, while the ending of the TFR is based on the actual end time. Or perhaps the sites are smart enough to scrape MLB.com's API to determine the true end time, I don't know.


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Why are pressure levels raised on warm days?
As I'm studying for the PPL knowledge exam, I had the following note:

Pressure levels are raised on warm days and the indicated altitude is lower than true altitude.

(source: AOPA)
But I don't quite understand why.
From my understanding, on warm days the air density is lower. If the air density is lower then the plane will think it's at a high elevation than it really is.
Secondly, if you have warm air and it can expand (since the atmosphere isn't a closed container), the pressure levels should decrease.
Hence, the entire statement that pressure levels increase on warm days and indicated altitude is lower seems false. But apparently it is true. Can someone reconcile this?

I think there may be a misinterpretation of words here: It doesn't say the pressure increases of warm days, in says the pressure levels raise. What is meant by this is: the geometric height of a surface of equal pressure level is higher on warm days than on cold days.

Does this answer your question? https://aviation.stackexchange.com/a/43475/19

Hot weather can affect flights. Example: Why Delta’s Athens-Atlanta Flight Had A Boston Stopover: https://www.youtube.com/watch?v=IYfJzsxR7kI

The formula:  PV = nRT is what many are taught to help understand the relationship between pressure, volume, density, and temperature.
It is very important to know, from an aviation point of view, how to apply this formula to an open, unenclosed system.
When air is free to expand Pressure stays constant, increasing temperature increases Volume.  This is what causes our "column of air" to grow taller. Because it is taller, it's pressure gradient from top to bottom is less per unit of true altitude, resulting in a higher pressure at a given True Altitude above a ground reference pressure.  The altimeter will give a reading lower than true altitude.
This is not to be confused with calculation of Density altitude on the ground for a given pressure and temperature. Because the higher temperature increases the height of our "column", it decreases the Density (same amount of air in a larger volume).
A hot air balloon is an excellent example of how this works.  Heating the air inside the ballon allows some of it to escape.  The pressure on the inside is the same as on the outside, but the Density of the heated air is much less, giving the balloon enough bouyancy to rise.

Remember, altimeters sense pressure rather than density. Pressure is essentially a measure of the weight of the atmosphere above a specific point.
You can imagine the pressure being measured as the weight of a column of air, 1×1 inch square at the bottom and extending up to the top of the atmosphere (around 100 miles). At the surface, that column of air weighs about 14.7 pounds. At 18,000 ft, that column weighs about half as much at around 7.4 lbs.
Thus, when the atmosphere warms, the column of air gets taller but weighs the same. Therefore, at sea level the altimeter wouldn't change. Let's imagine we're at 18,000 ft on a standard day with the altimeter set to 29.92 inHg. In this case, the altimeter reads 18000.
Next, imagine the temperature goes up. Therefore the column of air gets less dense and taller. Assuming the alt setting and height of the airplane stay the same, the height of the air column above the plane gets taller and thus heavier, leaving the altimeter to indicate lower than the actual height of the plane.

What is the difference between "atmosphere warms" and "temperature goes up?" In both cases, the air column gets taller but in one case the "air gets less dense" (i.e. in the case where the temp goes up)? Secondly, if the air column is taller but less dense, why would the weight be heavier? Wouldn't the growth in the column be equivalent to the loss in density, keeping the weight the same? I think this is where my confusion is stemming from.

And in general, I was told that if the temperature is really hot, the "density" altitude will indicate a higher altitude than true altitude. So does that mean this statement is false?

@Jonathan There is no difference between those scenarios. The total weight of the column of air hasn't changed. However, at the same true altitude, since the warmer column is taller, but less dense with the same total mass, a higher percentage of that mass will be above you than on a cooler day, thus higher pressure and lower pressure altitude. Your density altitude will indeed be higher, though. Altimeters measure pressure altitude, not density altitude. They have no method of measuring density.

@Jonathan the two cases aren't "atmosphere warms" and "temperature goes up", the two cases are "temperature goes up and you're measuring at sea level" and "temperature goes up and you're measuring at altitude".

Surely the column of air is not infinitely tall, rather it is the height of Earth's atmosphere above that point?

@reirab For some reason, your comment sort of made the click in my mind, thank you I get it now.

"the column of air gets taller..." Very helpful aid to understanding!  I've added it to my toolbox, thanks.

The relationship among pressure, density, and temperature is not simple. It is described by the ideal gas law:
$$ P=\frac{\rho k T}{\bar{m}}$$
where $P$ is the pressure, $\rho$ is the density, $T$ is the absolute temperature (in Kelvin or Rankine, not degrees Celsius or Fahrenheit), $k$ is the Boltzmann constant ($1.38\times 10^{-23}~\rm J/K$) and $\bar{m}$ is the average mass of a molecule of air (approximately $4.8\times10^{-23}~\rm g$ for dry air).
Notably, this contains pressure, density, and temperature. So it's not as simple as "if temperature goes up, pressure goes up," since it is equally possible that "temperature goes up, pressure stays the same, so density must go down."
By considering the forces on a parcel of air along with the ideal gas law, and assuming the air is not moving, we can find a pressure lapse rate $dP/dz$ that turns out to be given by:
$$ \frac{1}{P}\frac{dP}{dz} = -\frac{\bar{m}g}{kT}$$
where $g$ is the acceleration of gravity, approximately $9.8~\rm m/s^2$.
Notice that as temperature increases, $dP/dz$ decreases. In other words, the pressure changes less quickly with altitude. This means even if the sea level pressure is standard, the pressure at 1000 feet will be higher than usual. Or, in other words, the pressure levels 29.92 (corresponding to sea level) and 28.86 (corresponding to a pressure altitude of 1000 feet) will be farther apart. So if you climb to 1000 feet of true altitude, you haven't yet reached that level, and your altimeter will indicate lower than 1000 feet.

It's best to make this very simple.  If I'm five feet tall (and I'm not by the way), and I stand in a room that has a 10 foot ceiling, half of the atmosphere in that room is over my head.  If I move to a room with a 20 foot ceiling, 3/4 of the atmosphere is over my head.  If I am outside, on a warm day the temperature expands the atmosphere and the "ceiling" of the atmosphere goes up.  On cooler days the "ceiling" drops.  If you pick a fixed elevation, a larger or smaller percentage of the atmosphere is above that point depending on the overall height of the "ceiling" in relation to the fixed point.  The more atmosphere that is over that fixed point, the more weight of atmosphere is pushing down on that point, hence a higher pressure.  (Changes in air density due to temperature does not offset this in order to simply balance it out as was stated in a comment here.)

Suggested edit-- ". If I move to a room with a 20 foot ceiling, 3/4 of the atmosphere in that room is over my head."  May seem totally obvious from context, but one possible problem with the "room" analogy is that in the real world, all the atmosphere above the ceiling of the room still has an affect on the barometric pressure at floor level.  Your explanation still works, but best to make as clear as possible, so this tiny mod may help.

The mechanics of this analogy don't match up with the actual atmosphere in at least 2 relevant ways. First, air is far from uniform. At about 18,000' MSL, half the air molecules are below you, and the other half is spread out in the 100,000'+ above you, but the analogy posits uniformity. Second, if things got really cold & your ceiling lowered to 5'1", so barely above your head, you'd have almost no air above you in the analogy, and your altimeter would think you're really high and you'd dive to maintain altitude - the opposite of what actually happens. DV due to deeply flawed analogy.


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What exactly does "exercise extreme caution" mean when seeing alternating red and green light gun signals?
I've seen the light gun signals chart a hundred times but I don't quite understand when it says "use extreme caution" when you alternating green and red signals.
What does that mean exactly? Do I just treat the airport like a class E airport and just land based on my own discretion given I observe the pattern traffic? Or do I go around? Or do I have the option of doing whichever action I want to take (within safety realms)?

You wouldn't treat it like a class E airport, because it isn't.  Tower controllers are the ones emitting those light gun signals, and if you know what alternating red and green means, then you probably know what green means.  That's the one you are waiting for!  Good question though, I have wondered this myself because there isn't any clear expectation of what the pilot should do stated...

From FAA Order JO7110.65

3-2-2. WARNING SIGNAL
Direct a general warning signal, alternating red and green, to aircraft or vehicle operators, as appropriate, when:
NOTE-
The warning signal is not a prohibitive signal and can be followed by any other light signal, as circumstances permit.
a. Aircraft are converging and a collision hazard exists.
b. Mechanical trouble exists of which the pilot might not be aware.
c. Other hazardous conditions are present which call for intensified pilot or operator alertness. These conditions may include obstructions, soft field, ice on the runway, etc.

@randomhead- Sorry. I did a search, found a document on faa.gov and linked and quoted. I didn’t think to check the rev.

I'm still a bit unclear on what to do here. Would I just continue flying in and just be more alert or circle in position? Or am I supposed to expect another light signal after they issue the warning signal?

@Jonathan - The other light gun signals tell you what to do.  This one just says “But also be careful because there could be a collision hazard or you might have a mechanical problem or there may be another hazardous condition.”  I might first check a, b, or c depending on what phase of flight I was in when I saw the signal.


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If a plane doesn't require 100 hr inspection, but an AD must be checked every 100 hours, does that require a full 100 hr inspection?
Let's say a person privately own a plane and they are not using it for hire or instruction. So they are not doing the 100 hour inspections. However, there are several ADs that require it to be checked every 100 hours.
So when this AD is checked for compliance, does the mechanic also check everything else in the plane, i.e. is a proper 100 hr inspection performed?
Or, there could be several things wrong but it is not the responsibility of the mechanic if their main task is to just check the AD?

No.  If the plane is being inspected on an annual inspection basis, what gets checked on the annual gets checked on the annual, and if a special inspection is required every 100 hours by an AD, that gets done at 100 hours regardless of the calendar date of the annual, assuming the AD has no calendar requirement (they frequently do have calendar requirements as well as flying hours, to capture airplanes that are barely being used - like say, every 100 hrs or at annual inspection, whichever comes first, in which case you're goose is cooked).
You will use some common sense when it comes to scheduling. If you're 20 hours short of an AD compliance interval (with no calendar requirement) when the annual is scheduled, you'll probably do the AD inspection at the annual to reset its 100 hr clock so you don't have to have someone open the plane again a few months later 20 hours after the annual.
On the other hand, if it's 50 hours away with no overriding calendar requirement, you only fly 40 hours a year, and the AD is expensive and invasive, you might not do it at that annual if you don't have to, and will likely leave it until next annual.
The compliance language on ADs can get pretty complicated and sometimes you need a quasi-lawyer just to figure it out.  This happens a lot with transport airplanes.
In the case where you bring a mechanic to perform an AD-required inspection in between annual inspections, the mechanic only performs the work required by the AD (if the A&P who does the AD compliance procedure doesn't have an Inspection Authorization, they can't anyway).

If the AD inspection can be performed by a mechanic who couldn't perform the 100-hour inspection, that by itself is a pretty definitive answer to the question.

Great point @RalphJ


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Best way to correct for plane's natural tendency to turn left?
I am a new private pilot in training. I've only had about two hours of flight lessons so far, so I'm still quite new to this.
In a single engine, PA-28-181 Archer, I found that the plane has a natural tendency to bank left. I did some digging and people have noted this is due to the spin of the propeller and is natural.
But that meant that holding the control wheel in neutral position caused the plane to turn left so I had to constantly keep it tilted to the right.
My question is as follows:

Assuming no trim, should I constantly keep my foot on the right rudder to correct for this?
If I apply trim, will it correct for the roll or does it only stabilize pitch?

I noticed that at some points in my flight, when the instructor had me trim, the plane no longer veered left. But as soon I had to change attitude, the plane had a tendency to turn left and I had to re-trim. So I'm guessing the trim will correct for roll? But when I read about trim online, documents say that it only helps to maintain equilibrium attitude.
Bit confused on the proper/best procedure here. Any advice is greatly appreciated.

Always use rudder to keep the slip skid ball centered. Except when taking off and landing in crosswinds, that's all you ever do with it.  If the airplane rolls with the slip/skid ball centered, that's a bummer because it needs some aileron trim and there is only a fixed tab that you can't adjust until you're back on the ground.
For adjustable trim, the Archer has a trim tab on the stabilator surface for pitch/speed trim (doing double duty as an anti-servo tab to provide desirable pitch feel forces), and a bungee (spring) trim for the rudder control, operated by the black knob below the throttle quadrant by the passenger's left knee.  Here, we are only really concerned with the rudder trim.
The trim bungee provides a mechanical centering force to the rudder cable circuit that can be adjusted within a small range each side of actual center. When you're flying, you crank the knob to center the skid ball.
If the skid ball is to the right, requiring you to hold right rudder to center the ball, you crank the knob clockwise,  which squeezes a bit of right rudder for you so you can stop squeezing the pedal with your foot (you could take a bungee cord from the hardware store, hook it over the left rudder pedal, and pull some tension on it to apply some right rudder input - it's pretty much what the internal trim bungee is doing).
So in general, you just twiddle the knob as required to center the ball so you can relax your feet.  If the ball is off center, you can expect the airplane to want to roll away from the ball (ball to the right, left rolling tendency).
The Piper Cherokee family has a fairly strong rolling tendency when skidding, which is why they put the rudder trimmer in there in the first place; most light planes don't bother with rudder trim except for a ground adjustable only tab on the rudder that is normally bent (by trial and error) so as to center the ball in cruise flight.
When you have the ball centered nicely with no foot pressure with the trim knob, and you make speed/attitude changes, and power changes, it tends to change the equilibrium point in yaw so you will find that significant changes make the ball move from where you've trimmed it.
If the change is temporary and you'll be returning to your original power setting and speed, say in the next minute or two, you don't bother and just squeeze rudder with your foot knowing that you can relax it in a minute or two, but if the change is longer term, just turn the knob again to center the ball so you don't have to hold pressure with your foot.
So to answer your questions:

You should be always holding whatever rudder is needed to center the ball.  Without the Archer's rudder trim, you would have to hold this rudder pressure continuously except at one particular power and speed. So 3 cheers for the adjustable rudder trim.  Use the trim for any changes that are more than temporary so you don't have to squeeze the pedal all the time.
In the Cherokee family, keeping the ball centered will minimize rolling tendency, so it should fly straight hands-off if you have the trim set to center the ball. If the airplane rolls with the ball centered and needs aileron pressure to fly straight, in the normal cruising power setting and speed, the fixed tab on the aileron needs to be adjusted (bent up or down as required).

At some point your instructor will start having you practice using the rudder trim knob.  They're just waiting until you've internalized some of the basics first.

You really should talk to your instructor about this. Compensating for the inherent turning tendencies of propeller planes is a pretty basic skill that you should learn fairly early in your training. But, to answer your specific questions:

Assuming no trim, should I constantly keep my foot on the right rudder to correct for this?

In general, yes, that's the proper technique. Of course, your plane may be different, so check with your instructor or the POH.
Many small airplanes have a tab on the rudder that's supposed to do that for you when in cruise flight. If you find that you're having to maintain pressure in cruise, that tab may need to be adjusted. That's not going to be much help in other phases of flight, though, so you're still going to have to manually compensate.

If I apply trim, will it correct for the roll or does it only stabilize pitch?

Depends on what kind of trim you have. Most small airplanes only have pitch trim, which isn't going to help the turning tendencies any. If your airplane has roll or yaw trim, then, yes, those could be used to help.

To a first approximation, the most efficient technique is to apply right rudder as needed to center the slip-skid ball, and apply bank as needed to counteract any remaining turning tendency.
In most aircraft, if you keep the wings level and apply rudder as needed to hold heading, you'll get essentially the same result-- the ball will be centered or nearly so, and the aircraft will be flying in a straight line.


text:
What is the main rotor speed (RPM) of Sikorsky CH-53E Super Stallion?
In order to calculate the tip speed ($V_{tip} = ω×R$) of Sikorsky CH-53E Super Stallion, I need to find the rotation speed of the main rotor for the forward flight case. Can someone provide an answer with a reliable source? Also can someone provide the tail rotor RPM too?

The blade's tip can't move any faster than the speed of sound, at maximum forward speed. Its airspeed will be tip speed + forward speed. Have you tried calculating it back from that?

Typically a main rotor's tip speed is around  210m/s. With a radius of 12m this gives an $\omega$ of 17.5 rad/s i.e. 167rpm

@sophit That's seems rather slow. Most helicopters are in the range of 200-400 RPM. A Bell Huey is 320ish.  A 206 is around 400.  IIRC a Blackhawk is in the 250 range.

Just going by the published specs. https://sikorskyarchives.com/home/sikorsky-product-history/helicopter-innovation-era/sikorsky-s-70a-uh-60l-black-hawk-family/ Tip speed 725fps @258 RPM.

@JohnK: do you have something similar for the CH-53? Anyway if you scale 258rpm from  8 to 12m you get the (estimated) 170rpm of the CH-53.

Oh crap. When I skimmed over the title originally, "Blackhawk" went into my head, not Stallion.  Ooops. Your figure of 167 wasn't that far off.  Says here 185 https://www.thisdayinaviation.com/tag/sikorsky-ych-53a-sea-stallion/

It says here 185 RPM and 792 RPM for the A model.  For the E model with blades are 1.33 meters longer, you need to drop the rotor RPM to about 170 to keep the same blade tip speed. Note that those are nominal speeds; there will be a green arc RPM +/- range that brackets that value with a minimum and maximum RPM limit.

That's the value for the CH-53A (aka stallion) which has a different rotor than the E (aka super stallion) version: 11m radius and 6 blades vs. 12m radius and 7 blades. If we scale the rpm value according to the different radius, we get 170rpm that should be the correct value for the E version. @JohnK can you please update the answer?

Sikorsky CH-53E
Main rotor tip speed = 732 ft/sec
Main rotor radius = 39.5’
Main rotor rpm = 177
Tail rotor tip speed = 733 ft/sec
Tail rotor radius = 10’
Tail rotor rpm = 700
Vtip = (R x RPM) / 9.5493
RPM = (Vtip  x 9.5493)/R
Vtip  = ft/sec
R =  Rotor Radius feet
RPM = Rotor turns per/min
Ref: Helicopter Performance, Stability, and Control
Appendix B and pg.11
by  Raymond W. Prouty  (Author)
ISBN-10  :  1575242095
ISBN-13  :  978-1575242095
https://vertipedia.vtol.org/biographies/getBiography/biographyID/88


text:
Can I become an aircraft technician if I have color-blindness?
Are there any restrictions imposed by any aviation-authorities that would prevent a color-blind person from becoming an aircraft technician?

Could you indicate for which country you ask?

@U_flow OP tagged both FAA and EASA so it seems like this is a general question.

It will depend on maintenance tasks.
The FAQ page for Part-66 licences has this information:

Regulation (EU) 1321/2014 does not require any medical examination before applying for a Part-66 licence.
In the past some medical criteria were proposed in JAR-66, but these were removed in order to avoid conflicts with national rules. JAR 66.A.50 had requirements on:

use of alcohol at work,
effects of medicines,
physical conditions, vision, ability to see colours,
mental conditions.

However, the same pages also states:

Common sense recommends the certifying staff not to exercise the privileges of their certification authorisation if they know or suspect that their physical or mental condition renders them unfit to exercise such privileges (impact to the safe maintenance operations). ...

Please find an agreement with your company in order to list the maintenance tasks that you are allowed to carry out without jeopardising the aircraft safety.
Note: The same reasoning as explained above applies for any medical condition.

While it may be true that having a poor color vision or even color blindness may not prohibit some of the maintenance tasks, there for sure are many in which color vision is a necessity, such as electrical work. In many jurisdictions one cannot even become a household electirician with color blindness, as wires are color coded.

Another example where this would be problematic is when dealing with lights on the aircraft. Installing the wrong wingtip light covers because you can't tell the difference between red and green would be problematic, for example.

In general, aircraft wiring is not color coded.  Just lots and lots of white wires.  Unless it is test equipment, then orange.

Yes absolutely, you can become an aircraft technician if you are colorblind. The European Union Aviation Safety Agency (EASA) even has an FAQ for this in which someone asked "I am a colour-blind. Does this prevent me from getting a Part-66 licence or exercising my licence privileges?". The answer states that

Regulation (EU) 1321/2014 does not require any medical examination before applying for a Part-66 licence.

Note that you need the EASA Part-66 licence in order to work on "Maintenance on aircraft structure, power plant and mechanical and electrical systems" etc.
Therefore you do not need to see color to become an aircraft technician.
The same applies for the FAA in the USA.
If you google around a bit, you will see that color-blind aircraft technicians share their story on relevant forums.

Depends on the country, but most do not require a medical certificate to qualify for an aircraft maintenance certificate. Your sight would not be tested.

Passing a certification is not the only requirement to do a job.

@user253751 That is not relevant to the question. Becoming an aircraft technician is one thing, getting a job that has vision requirements is another. That is an issue with the employer, not the country's aviation authority.


text:
Why do helicopters need two pedals? Why is one not enough?

Picture source
Why do helicopters have two pedals (as shown in the picture); why is one not enough? As we know, the pedals provide directional control through the tail rotor, to yaw left or right. As we also know, the tail rotor is required to overcome the torque of the main rotor, which will otherwise cause the helicopter to spin. So, in my opinion, one pedal is enough: increase the tail rotor's pitch to yaw against the main rotor torque, and decrease it to yaw following the torque direction. My question is then, why do helicopters need two pedals? Is one not enough? And how exactly does the pilot control the helicopter in yaw?

Your question seems unclear to me. You are correct in saying that the pedals increase or decrease the pitch of the tail rotor. One pedal to increase, the other to decrease. They are linked up in just the right way so that left pedal -> turns left, right pedal -> turns right. I don't understand how you got from there to "one pedal should be sufficient". See https://pilotinstitute.com/helicopter-controls/

How do you propose the single pedal travel back? By spring loading it back? Then what happens when the pilot's foot accidentally slips off the pedal? We could make a car use one pedal, but when your foot slips off, your car squeals to a stop with maximum braking.

Perhaps you could describe your solution in more detail.  With just one pedal, let's pick pedal 2 in your picture, how would your foot operate it in both directions?

Comments are not for extended discussion; this conversation has been moved to chat.

I'm not sure I understand the downvotes. The question is clear, specific, on-topic and novel. The answer might seem obvious to a pilot but obviously not to the OP and we shouldn't judge a question by how obvious we think the answer should be, should we? Or am I missing some other factor? Personally I feel compelled to apologize to the OP on behalf of the community. Also +1 from me.

Two pedal control is arranged such, that, in helicopters and other aircraft as well, the pedal forces are balaced (by default or by trimming) during straight and level flight.
The arrangement suggested in question would mean that the pilot would need to apply positive force on the single pedal all the time. This will quicly become strenuous, and maintaining a constant force for a long time is not as easy as one might think.
Physical strain is not the only downside. One pedal arrangement cannot be trimmed for neutral condition, as this would remove the ability to use full control envelope. This leads to extra "mental" load, as maintaining directional stability needs constant focus.
In addition to ease of use, the two pedal arrangement assures the control forces will not get excessive. With two pedals the range of the requiered force can be described as, say, 0 to 100 units for either foot from one extreme to the opposite one. With one pedal, the range would be 0 to 200 respectively, or twice the travel of the pedal to compensate for this.

Even in std configuration there must be a trim for both pedal position and pedal force, since hover and steady forward flight are both common operating points, with significantly different rotor torque and hence tailrotor thrust. I don't see why a single pedal arrangement should not be capable to be trimmed.

@CarlBerger One of the requirements of trim is to have zero stick and pedal forces. If you have one pedal, it will have to be spring loaded, making it untrimable. And if you remove the spring, you suddenly lost the ability to yaw in one direction; the only way to yaw in that direction is to use the upper surface of your feet to pull the pedal back. This makes trimming with one pedal basically impossible.

sure you would need to push & pull it from the neutral position. Technically, a simple fabric loop around the pilots boot should suffice to pull. I agree that it wouldn't be very practical

@AdityaSharma, helicopter flies less frequent make yaw compared to a car make turn. In an automatic transmission car, it just involves one leg to drive: Right leg only. The intention to do so is to reduce the driver's fatigue when driving especially when driving long distance (long time). Why does the same thought can not be implemented in helicopter? By one leg idle, the pilot will les fatigue. To synchronize the legs are not easy, even we can do after some training. Try automatic transmission car, you will se the different.

@AirCraftLover This car logic doesn't apply on helicopters, since pedals on helicopters serve a completely different purpose: yaw control. Helicopter pedals are similar to the rudder pedals on aeroplanes - to yaw left, you press the left pedal; to yaw right, you press the right pedal. To go straight, you press no pedal. One pedal increases tail rotor pitch, and the other pedal decreases it. If you press no pedal, the pitch is in between and the helicopter doesn't turn.

As you said, the helicopter flies straight most of the time. And as I said, to fly straight, you don't need to press any pedal; you only have to press a pedal when you have to turn. So you can see that having two pedals actually reduces the fatigue, since you don't have to press any pedal most of the time. It is also simpler to fly with two pedals.

"The arrangement suggested in question would mean that the pilot would need to apply positive force on the single pedal all the time." Not really that way, could be done a mechanism to auto-lock the pedal to its 'neutral' position, unlock and press or release to move to the intended direction. Bear in mind that pedal used or yaw, not for banking, which it is not so many times yaw is done. To make turn, a helicopter used cyclic rather than pedal.

@AdityaSharma, pedal is not the same as to control rudder in airplane. Learn again about helicopter control. Banking or making turn is using cyclic, not tail rotor.

There are of course other ways you could control the tail rotor, for instance there is a control adaptation for disabled people where the tail rotor is controlled by hand rather than foot using a yoke on the stick rather than pedals.
A single pedal system is possible, but it would be either unsafe, uncomfortable to use, or both. There are two viable options for a single pedal system (there are more possibilities but none are viable in a direct linkage system where significant force is involved). It's not a problem of intuition, there's no reason people couldn't use a single pedal with precision, it's mostly to do with musculature.
Spring loaded: in this scenario there is a spring to force the return of the pedal. There are several problems with this:

Safety: if the spring breaks then the pedal will not return, the torque will remain in the last setting, which is potentially fatal. I would not want to trust my life to a single spring
Muscle fatigue: there's a reason cruise control was invented for cars, holding a pedal down for long periods of time is tiring. I've driven across the US twice in a car with no cruise control and I can tell you it was very uncomfortable, there were times I had to stop just to give my calf a break. That was in a modern car with sensor based throttle control and a light pedal, in a helicopter with direct linkages and no power assistance the forces involved would be higher, and much harder on the calf muscles. This could be mitigated somewhat with friction locks for cruise, trims and the like, however it would still be a significant factor

No spring: In this system the single pedal would be pushed down and pulled up by the foot, which would either be strapped to the pedal or more likely there would be some sort of hook over the toe. The overriding problem with this design is that the human leg isn't built for it - the calf muscles are far stronger than the muscles on the front of the leg. Try lifting even a small weight with your foot and you'll find it's very hard to do, and your muscles will quickly tire.
There are other possibilities, for instance side to side motion, however those muscles are even weaker, you would not be able to use that in a helicopter with a direct linkage.

You're holding collective with one hand, cyclic with the other; how do you control the tail rotor with hand?

I wanted to include a picture @AdityaSharma, I couldn't find one that isn't copyright unfortunately. There's good pictures in this Wired article: https://www.wired.com/2009/08/hand-control-airplanes/

Understood, thanks for reply. Controlling all three axis with one hand - that must require some serious skill!

@AdityaSharma give the pilot 2 yokes - left hand does collective and yaw, right hand does cyclic. This is infact the way it's done in most model helicopters, the remote controls don't have pedals ...

Flying a helicopter at all requires serious skill @AdityaSharma, I actually don't see hand controls being that much more challenging.

@GdD Indeed it requires some real skill to fly a machine that spins out of control if you let it do what it wanna do. Can't really say if hand controls would be more difficult or not, since I haven't flown a helicopter before... Good day!

https://www.semanticscholar.org/paper/An-investigation-of-side-stick-controller%2Fstability-Aiken-Hilbert/a575742db358b1256eacf81e69d805f524c3962c/figure/1  Shows some control configurations which have been investigated.

@GdD, I use automatic transmission car. It only involves my right leg, my left leg not involve. With automatic transmission, I can drive hours without feels fatigue, compared to when I drive a manual transmission car which involves my both legs. The same thought to me, why such way can not be done with helicopter? Of course will use something like spring to return to neutral position, which neutral we assume the position of zero torque. How to do single pedal, I think it is not a hard mechanism, it is very easy instead.

No @AirCraftLover, it would not spring back to neutral, it would spring back the other way fully, there would have to be constant pressure on the pedal to maintain neutral torque.

We have two directions in yaw, and we have two feet, so we can afford to have two pedals - one for each direction.

One pedal? Theoretically, yes. You can spring load that pedal to make it serve the purposes of both - press to yaw left, release to yaw right, partially press to maintain heading. But I can imagine myself getting confused all the time - if I gotta turn left, do I press or release? - not convenient when you need to make a sudden input. With two pedals, you don't have to think - you simply press the pedal corresponding to your intended yaw direction. Single pedal creates more problems than it solves, and it solves none.

Did you really read my post before you commented? You wrote your comment just exactly like I wrote on my post.

@AirCraftLover I fully read your post. Yes, it's possible to have just one pedal control the yaw. By that logic, you can bias the rudder of an aeroplane in one direction, and then you only need one rudder pedal! But imagine how ridiculous it would be to have one rudder for two directions, plus you will have to press that rudder all the time, which won't be fun.

It's simply more intuitive to have a rudder pedal for each direction. If you suddenly have to yaw left, what is more intuitive? Wasting time deciding weather to press or release that one pedal, or just flooring the left pedal?

Not exactly press one pedal or the other but balance the two.  Certainly in IMC you’d want to align your feet and expect to go in a reasonably straight line.

@AirCraftLover I've completely modified my answer, let me know what you think of it now.

On the other hand: single stick controls four directions in that sense, with no problems.

@Jpe61 You can both push and pull a stick, but you can only push the pedal, not pull it.

of course you can pull a pedal - see bike pedals other than platform: eg clipless or clipped. Also e.g. gear shifting on a motor bike is done pushing and pulling the lever using the left foot

@AdityaSharma one could easily make a pedal which can be pulled. Older bicycles had these for example.

@CarlBerger Theoretically, it is possible, but for something as critical as yaw control, you definitely wouldn't want having to constantly manoeuvre your feet around the pedal. It would be difficult to control it precisely, and not ideal when you need to make a sudden input in an emergency. :)

@AdityaSharma, automatic transmission car just involves right leg to control throttle and brake as they work different time. Yaw also work in different time. Please be noted that pedal is tool for yaw, not to make turn. To make turn during fly is using cyclic. To make left turn, cyclic is left, to make right turn, cyclic is right, the same way to do moving forward and backward.

@Jpe61, yes, exactly. Yaw left and yaw right will be done in different time and never be at the same time, so why must need two pedal? Just like automatic transmission car, it just involve right leg only so reduce fatigue of the driver. The same way should be able to be done with helicopter.

@AirCraftLover forces involved with aircraft pedals is very different from that of a throttle or brake in a car. Also switching between pedals in acft would be a huge risk, imagine pressing the wrong one...

It's true that yaw control is only one axis, so a single (1-axis) control lever, as in 1 pedal, would theoretically be enough. In standard helicopter design, the two pedals in a helicopter also only actuate 1 axis (yaw), and sometimes (usually?) they are even fixed to a bar that rotates around a single center pivot.
While I'm not a helicopter controls designer, I assume the main reasons for that are:

History -  Helicopters became airworthy after airplanes had been common for a while. Autogyros date to 1923, and some helicopter development seems to have taken place in the late 1920s to the late 1930s, with the first mass produced helicopter being Sikorsky R-4, in 1944 (https://en.wikipedia.org/wiki/Helicopter#First_practical_rotorcraft). Fixed wing airplanes were a lot earlier, e.g. the Blériot XIII  dates to 1908. It was this airplane that was using the yoke + rudder pedals controls, and in airplanes, this type of control was to stay. It's not very far fetched for a new design, to use proven controls that pilots are used to - especially in the early testing phases.

Controls precision - There is a reason we use our hands to write, or to operate machinery. A kettledrum may be operated per pedal, but a violinist usually doesn't stomp on the strings (unless angry?).  Most humans are quite capable of doing precise complex movements with their hands, and a lot less with their legs/feet. Feet-operated controls are usually the ones that require a lot of force but are not super delicate, like a brake pedal in an old car without power brake. Using both feet to pivot a single bar for yaw makes precise control much easier.

force symmetry: in case of maneuver loads (say, a +3g turn), in a two pedal design both pedals get the same force, and they cancel each other out. In a single pedal design, whether push-pull or whatever, it's the pilot who needs to counter-act those forces by muscle strength if no control input is desired


text:
Why do turbine rotors have an outer shroud but compressors don't?
I know it's purpose is to prevent interstage leakage of pressurised gases, but then shouldn't the compressor stages have it as well? Why is it exclusive to the turbine stages?

Possibly related: https://aviation.stackexchange.com/questions/80717/why-dont-turbofan-engines-have-shrouds-affixed-to-the-fan-blades

Thank you for the input, that's an interesting question - one that I've had in past.

The shroud fulfills two purposes:
It improves aerodynamic efficiency at the blade tip (reduced tip leakage, due to better sealing possibilities with fins etc.), and it also changes the blade's vibration eigen-modes, because the shrouds interlock. The single blade dynamics changes to blade dynamics of the whole row, the blades get coupled.
Some more elaboration on why some turbine blades have shrouds and others don't, i.e. issues with shrouds:

Centrifugal load: the weight of the shroud needs to be carried by the blade, under significant centrifugal load, and at high metal temperatures (often close to some creep limit design criterion)

Blade dynamics: adding weight to the tip of the blade lowers the eigenfreqency significantly, and also changes other dynamic mode shapes - and you need to stay outside excitation frequencies.

Cooling: depending on the turbine stage, the shroud will need cooling as well, so there is a trade-off whether the increase in stage efficiency is enough to justify the increased cooling air consumption, and how much is left in the end

Why compressor blades usually don't have shrouds:

Blade dynamcis: Compressor blades are usually already build as thin as possible (for aerodynamic efficiency), while maintaining a minimum thickness for mechanical stability and vibration dynamics. Adding a shroud would be quite a challenge to blade dynamics.

Tip leakage: in compressors, the thermal deformation between hot and cold state is not as large in compressors as in turbines, which means that the tip gap can be designed tighter. Secondly, in compressors the pressure ratio per stage is a lot lower, so the tip leakage isn't driven by as much pressure ratio. That said, to my knowledge, the tip leakage is one of the main limiting factors modern compressors, both for efficiency and compressor stability / surge margin

You have cleared all my doubts, thank you!


text:
What is the number in a closed box right below a victor airway?
I see this little number in my TAC.

What is that "42" inside a box right below v208-458? I looked through charts guide to see if I can find that symbol but I couldn't find anything (pretty sure I missed it). Does it have something to do with victor airway or is it separate?

Per the Chart Users Guide (which covers sectionals and TACs), that is the total mileage between navaids.


text:
What are the different ways my alternator is failing?
I am a student pilot training to get my PPL. My CFI asked me the following:
How can you know your alternator stopped working?
My first response was annunciator lights. Then the instructor said, what if you don't notice the annunciator lights or they themselves aren't work, what's your next check?
Check the amps. But then I started wondering, the plane will most likely have some amount of battery power. So the amps might not be fully gone.
Are there other ways to determine if the alternator isn't working?

‘Check the amps’ is an eyebrow-raiser; amps is not a thing that you either have or don’t have.  There’s not much point in having an ammeter if you don’t know what it’s showing you.  Typically it will indicate overall current to/from the battery, which should be slightly positive in normal operation but will go negative if the battery is discharging.  However, an ammeter could show the output of the alternator, current being drawn by certain instruments ir equipment etc.

If you have a voltmeter installed, a failed alternator will cause it to read ~12V (or ~24V) while a working alternator should show ~14V (or ~28V)
The voltage regulator (wired in after the alternator) outputs a voltage higher than the nominal battery voltage so that the battery can charge. If the voltmeter reads the nominal battery voltage it may indicate a failed alternator. It can also indicate a bad voltage regulator.

Depending on the type of ammeter your aircraft is equipped with it may show discharge vs charge current in which case a discharge indication could indicate a failed alternator or an alternator that can no longer supply the required current to operate all of your electronics and you are operating at a loss.
Lots of good info here if you are curious.

Load meters only show the actual output of the alternator. They don’t really show “total consumption”. If you turn on an item, like a landing light, it will show as a load on the ammeter, meaning the alternator is working. If there is no load, the alternator is not working, or there is nothing turned on.


text:
UK noise certificate for below 9000kg?
This is regarding UK regulation - The Air Navigation (Noise Certification) Order 1990. I am curious what is the reason of specify this particular upper weight limit here:
"every propeller driven aeroplane having a maximum total weight authorised of 9,000kg or less;"?
Just a bit below there is another point:
"every other subsonic aeroplane which in accordance with its certificate of airworthiness has a take-off distance required, at maximum total weight authorised on a hard level runway in still air in an International Standard Atmosphere at sea level, of more than 610 metres;".
I suppose there is some class of aeroplanes that somehow falls between these two categories (what is it?), otherwise what sens would that inclusions/exclusions make?


text:
Who sells aviation fuel to airports?
I want to ask about the jet fuel purchasing cycle. Is it the airport that buys the jet fuel from refineries and then sells it to the airlines? Or do airlines buy fuel directly from refineries?

At least at our local airport (KGRB), there are two FBO's that service the airlines. They purchase the fuel from wholesalers who truck it in, and then they have contracts with the airlines to refuel them. Larger airports have on-site tank farms and I believe they are owned by a fuel servicer that runs at the airport.

Airlines hedge against fuel price fluctuations in the financial markets, but buy the fuel locally at local prices. Both are not connected.

At the FedEx hub in Memphis there is an underground pipeline directly from the Valero refinery a few miles away

The airport is buying fuel and selling to airlines. And more over, they provide different types of fuel on prior request by the airliner. Reference : VCBI

This may be true in Sri Lanka but I would avoid making a blanket statement because its not the case in the US.

Airlines make direct contracts with the major petrol companies eg Chevron, BP, etc. for large quantities of JET-A, often years in advance of its use.  It is the best way for them to buy fuel in bulk like that and it is traded much like stocks are.  Airports themselves will fuel airliners with on-hand supplies of JET-A and be reimbursed for it by the petrol companies.

It depends. Airlines have contracts either directly with say Shell or they have a contract with a 3rd party that handles their fueling requirements.
That 3rd party may or may not be the airport, it could be another company servicing the airport (and maybe others as well).
And such contracts may well differ depending on location. E.g. at their home base(s) they may draw directly from Shell, but at remote locations where Shell does not supply fuel to they may buy fuel from Joe's oil and lube supplies.

authors: Александр Федоров

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why is the F-35 engine nozzle exposed?
I was wondering why the F-35 engine is exposed. The F-22, F-16 and the YF-23 Black Widow have the exhaust nozzles of their engines blended flush with the fuselage.
(YF-23 Black Widow)
(F-35)
The YF-23 Black Widow clearly has two engines which are indented into the plane. However the F-35 has one engine which is exposed at the back. So I have two questions:

Why does the F-35 have only one engine unlike the older models?
Why is the F-35's engine exhaust nozzle clearly more exposed to bullets and rockets than older models?

The two questions are not really related to each other.

Re: #2, Being more exposed to enemy fire is not really relevant. Airplane skin is not armored; in fact it's as thin and light as possible, and does nothing to protect against bullets or fragments.
Combat aircraft generally carry no armor whatsoever except for (sometimes) around the cockpit to protect the pilot; the cost in range and maneuverability far outweighs the benefits of increased survivability.

The F-22 is intended (and the competing YF-23 was intended) to be an air superiority fighter to replace the F-15.  The F-15 was a twin-engine model, as was the F-4 Phantom that it replaced.  Twin engine planes have excellent "survivability," meaning they can take a lot of hits and still stand a chance of making it home (you can lose an entire engine and still stay airborne).  They also are much higher-performance.  The F-15 can takeoff, stand on its tail and climb vertically to over 40k feet.  I've seen F-16s do something similar but they couldn't climb as high and they couldn't carry as much fuel and/or armaments while doing it.
The F-35 is intended to be a replacement for the single-engine F-16.  The F-16 is a capable workhorse but it's more of a budget, multi-role aircraft, meaning it was designed, from the get-go, to do some air superiority AND some ground attack, operating on a tighter budget.  We have considerably more F-16s in the Air Force inventory than we've ever had (or will have) F-15s and/or F-22s.
The F-15 used the same engine as the F-16 (two to choose from: either the P&W F100 or the GE F110), but two of them.  When I was in the Air Force

F-15's cost about USD 50 million
F-16's cost about USD 35 million

Two engines and a larger, heavier airframe (albeit less sophisticated) added up to a significant price premium.  In air combat, above 25k feet, my money would be on the F-15.  At altitudes below 25k feet, my money is on the F-16.  With an inherently unstable airframe and a computerized fly-by-wire system to tame it, the F-16 was wickedly maneuverable.  The increased power of two engines on the F-15 working against the greater weight really loses the advantage at lower altitudes.
The "buried" engines in the F-22/YF-23 are trying to improve the stealth characteristics.  The F-22's thrust vectoring, coupled with twin engines, make it more stealthy than the F-117 and faster and more maneuverable that the F-15 or the F-16.  The YF-23 was arguably faster, but less maneuverable.
The F-35 is intended to cost less than the F-22 (debatable as to whether that will be the final case).  And certain models of the F-35 are capable of a trick the F-22 simply can't match: STOVL.  It can do a Short TakeOff and Vertical Landing.  For the latter, the rear nozzle pivots downward 90% and a "lift fan" opens up in the forward fuselage.  The F-22 was never intended to be able to do this.  "Burying" the rear nozzle AND trying to do that would add a LOT of weight (and additional cost) to the aircraft.

debatable whether [the F35 will cost less than the F22] - Just FYI, the incremental cost for an F22 was about $150 million. F35 prices are still falling. The latest A-models in LRIP lot 9 cost $98 million flyaway (including engine). It's on track to fall to <$75million by 2019. Average cost: <$80 million per F-35A over the entire production run. All prices in FY15$.

What about the F35-A and -C? The F35-B is only capable of performing STOVL.

True, only one of the three does STOVL. But, like the F-16, it's intended to be the "lower-cost" model.  Adding extra fairings to "bury" the engine would add weight (hurting the performance) and increasing the cost.

@HugoWoesthuis - A model doesn't need STOVL. It operates from conventional strips. The C model doesn't need STOVL because it operates from CATOBAR carriers. Neither model should have STOVL.

The answer to both of your questions lies in the vertical takeoff and landing (VTOL) capability of the F-35.
In order to provide vertical thrust, the rear part of the engine rotates downward. This is much easier to accomplish if the engine exhaust is not kept on top of the plane, as in your YF-23 example. This rotating ability is also much easier to accomplish if there is only a single engine. With two engines, they would both have to rotate downward and drive the lift fan together. The Boeing X-32, which was competing against the X-35 for the Joint Strike Fighter contract, also opted for a single engine design for similar reasons.
Your impression that the YF-23 protects the engines more doesn't really apply to things like bullets. Although the engines on a fighter are buried more in the fuselage than a typical modern airliner, they are still not protected much from the top or bottom, regardless of the configuration. If shrapnel is hitting the plane, protecting the engines will not solve all of your problems. You will notice that planes like the A-10 that specifically are designed for withstanding damage do not go to great lengths to hide the engines. Planes like the B-2 and YF-23 hide the engine exhaust for stealth reasons.

Given man portable heat seeking missiles, reducing the heat signature seen from the ground is how you go about protecting the engines. People don't shoot down planes with bullets that often.

Why does the F-35 have only one engine unlike the older models

The USMC wanted the aircraft to have S/VTOL capability. This meant that the aircraft should have a single large engine as the best configuration would be to have a single engine with swiveling nozzle aft of center of gravity for supersonic flight.
Having multiple engines in a VTOL aircraft is problematic- The same reason Harrier and Yak-36 has single engines (Yak-36 had smaller engines purely for takeoff, but had an automatic ejection mechanism). Loss of a single engine in a multiple engine configuration will result in uncommanded roll during (vertical takeoff and landing).
The F-35 engine is one of the most most powerful military engines out there. Improvements in engine technology have made the aircraft engines more reliable compared to the older models, so dual engines are not a requirement anymore except in some special cases.

Why is the F-35's engine clearly more exposed to bullets and rockets than older models

I think you're talking about the nozzle being visible. All modern combat aircraft engines are buried in the fuselage for various reasons. The nozzle is exposed in case of F-35 as it has to swivel for VTOL.

Source: jeffhead.com
Note that this configuration is similar to Yak-141 (which again had two lift engines), another VTOL aircraft (Lockheed Martin worked with Yakolev during the JSF competition).

F-22 nozzle is exposed- This is a result of the thrust vectoring. YF-23 and other stealth aircraft (like B-2) had nozzles above the fuselage to reduce IR signature from below.

"Loss of a single engine in a multiple engine configuration will result in uncommanded roll during (vertical takeoff and landing)" - Not if the two engines exhaust through a single nozzle.

The exposed engine outlet nozzle of the F35 is a serious shortcoming of the design, it makes the F35's IR signature much greater than that of the F22 and significantly increases it's vulnerability to IR guided weapons. Cost and weight factors probably played a part but in engineering terms it would be difficult to fully enclose the vectoring nozzle of the F35B version, necessary for VTOL operation.


text:
What measures are taken to ensure military pilots aren't going mad?
I'm asking specifically about single pilot fighter jets. How do air forces ensure pilots won't go mad and start a war with another country? For example, a pair of B2 pilots could start World War 3 by attacking China.
Are pilots checked regularly by a psychologist?

why the downvotes ?

You seem to be assuming that if a single pilot did lose it and attack another country, the response would be immediate all-out war. I suspect that today, a more realistic response would be a flurry of diplomatic activity leading to a negotiated apology and possibly some minor reparations. The basic question about psych screening for military pilots seems valid to me, but the premise behind it may not be.

Like any other military officers, pilots are under scrutiny of their fellow squadron mates and their superiors.  Such drastic behavior would leave serious red flags prior to culminating in an event like that and said pilots who exhibit such warning behavior would probably be relieved of flight duty pending a Performance Review Board and psychological investigation to determine continued fitness for flight duty.
In addition there are quite a few safeguards in the nuclear chain of command to prevent an unauthorized use of nuclear weapons, including a verification process to arm the nukes which requires the consent of both the pilot and mission commander to authorize nuclear delivery.  The strongest deterrent is that, during peacetime, very rarely do aircraft fly with live nuclear weapons in their bellies.  This is primarily done to prevent an incident like the Tybee Bomb or the Goldsboro Incident from happening again.

Depending on what their responsibilities are, a pilot may require a security clearance. For example, from the US Air Force Reserve website the requirements to pilot a B-52 include:

Bachelor's degree; an undergraduate degree specializing in physical
sciences, mathematics, administration, or management is desirable
Commission as an officer in the Air Force Reserve
Current aeronautical rating and qualification for aviation service as
pilot
Eligibility for Top Secret security clearance *

*emphasis mine
Security clearances are for keeping sensitive information out of enemy hands, but they also serve to protect the public and the country from harmful or erratic behavior. In order to get a security clearance one must pass background checks and answer psychological questions. Sometimes they also require a polygraph.
When someone has a security clearance it is the responsibility of their superior officer to monitor for signs of mental problems. The officers are given guidelines and lists of clinical indicators to watch for.

In general, clinical indicators of the greatest concern are those that
are disruptive to an individual's work or social functioning and/or
activities of daily living. These indicators are sometimes identified
during investigative interviews with supervisors, coworkers, friends,
and neighbors.
source: Military.com

Depending on the type of clearance they hold, they will also be subject to periodic reinvestigation for their security clearance.

A periodic reinvestigation (PR) is when a currently cleared individual
is required to review, update and resubmit his or her security
clearance application for a reinvestigation. Periodic Reinvestigations
are routinely required every 5 years for those with Top Secret
security clearances, every 10 years for those with Secret security
clearances and every 15 years for those with Confidential security
clearances. A PR is done to ensure that a candidate should still have
access to classified information. Random PRs are also administered at
the discretion of the granting federal agency.
source: ClearedJobs.net

If problems are suspected there are different ways that can be chosen to deal with them. There's always a possibility of someone not exhibiting signs or managing to hide their behavior. But there are certainly protocols in place in the military to make sure that an unstable person is not put behind the controls of a weapon with deadly capabilities, not to mention the huge financial cost of the aircraft and systems.


text:
Image recognition in air to air missiles
Given advancements in machine vision, what issues would there be if air to air missile guidance used image recognition (at least in the terminal phase) to defeat flares and chaff?

Night.  Clouds/fog.

Modern missiles are using computer vision or image processing technology.
Obviously, for this to work you need an image, which is typically provided by an image-generating IR-sensor (note that this does not work with radar sensors of course). In the past, IR-sensors would only try to determine the brightest (hottest) spot in the sky. This technique is of course susceptible to counter-measures such as an IR-flare or trying to get the missile to lock onto the sun. But an image of course contains much more information which can be used to improve guidance and differentiate counter-measures. Current seekers try to use the information contained in the monochrome picture to extract more information and discern countermeasures from the target etc.. In the future multi-band imaging sensors will be used in order to gain even more information, e.g. a color picture vs a monochrome picture.
One good example which uses image recognition today to improve its end-game capability, is the (now famous) Diehl IRIS-T:

Intelligent image processing detects IR decoys in the image and ignores them.[...] For identification purposes, there is a target library that contains images of all known military aircraft from different perspectives. For each target, eight of the most vulnerable points are saved, towards which the guided missile is heading.

Translated to english, from the german IRIS-T wikipedia article (I should note that not all links in that quote are available anymore).
This is known as "aim point selection" which can only be used, if you have a somewhat advanced understanding of the image which your seeker currently sees. Other missiles such as the AIM-9X Sidewinder or the ASRAAM also use imaging IR-Sensors, and most likely employ similar techniques as the IRIS-T.
And this apparently works very well:

Buschek said Ukraine had shot down more than 110 targets, most of these cruise missiles such as the Kalibr, with a hit rate of almost 100%.

(source)
You can get a good sense of how such an engagement looks from the missile side in this paper:

Re, "this does not work with radar sensors" Specifically, it does not work for acquiring and tracking targets for anti-aircraft missiles, but you can use much larger radar antennas to make interesting images of much larger targets. https://public.nrao.edu/news/radar-tycho-crater-intricate-detail/

It is used by modern missiles. Military isn't the birthplace for every tech, but they've been working on image recognition long before the general public heard the term "image recognition". This 1989 article, barely 35 years old, elaborates on some of the earliest neural networks used.
Shape is but one of many factors used by missile seekers to differentiate the true target. Dynamics and behavior are just as important. This is because flares are very bright, and the image in question very low-resolution.
Different guidance techniques for the approach phase and the terminal phase is outright 50 years old, since the second generation of contrast seekers, though early systems just went for the image center.

Let it not be unsaid that neural networks and computer vision aren’t inseparable; when I published a paper on the latter in 1987 there was already a wealth of techniques and at that time most didn’t use neural networks.

Military research into projectiles guided by image recognition appears to date back to the 1940s.


text:
Why are aerodynamic models without stall used in research papers?
Some papers that I'm reading in the Journal of Fluid Mechanics give aerodynamic models in which the lift force appears to not take stall into account -- unless I am misinterpreting the math model, but I don't think I am.
Is it reasonable to use a model without stall?  What would be the advantages of doing so?

Because they don't need to. Scientists and engineers always have to trade-off between efficiency (speed, computational resources needed,...) and accuracy when doing calculations. If the calculations to be performed are not expected to be in the stall region, it's easier to use simple models without modelling the stall behaviour.
It's not only about doing more calculation steps but also about the methods you can apply. For example, the lift coefficient $C_L$ is often defined as a function of the angle of attack $\alpha$:
$C_L= C_{L\alpha} \cdot \alpha$.
This equation doesn't take stall into account and can lead to infinite lift coefficients. A slightly more accurate formula, which limits the maximum lift coefficient, is the following:
$C_L = C_{L\alpha}\cdot \sin\alpha$.
Why don't we always use the second one? Computing a sine isn't that expensive, isn't it? Well, for example, control engineers usually use state-space models, which require linear terms. Also, the derivative of the first formula is a constant, while the derivative of the second one is still a function of $\alpha$. Doing mathematical calculations or deriving formulas is much easier using the first one.
Obviously, care should be taken to not use the simplified formulas in design regions where they are not valid.

Also, linear models make it easier for simple matrix solvers to converge.


text:
In a hot air balloon what proportion of the lift comes from the heated air compared to that from the combustion products?
A hot air balloon is heated with propane or similar hydrocarbon. One of the products of combustion is H2O with molar mass 18 so it is substantially less dense than air (78% nitrogen, molar mass 28 and 21% oxygen, molar mass 32). Another combustion product is carbon dioxide, molar mass 44. Does the balloon get much (or any) lift from the combustion products or does it all come from the higher temperature? A supplementary question: does the water condense on the envelope thereby increasing its weight and reducing lift?

I think that on chemistry.se you'd get a better answer

There is no lift (aerodynamic force created by a relative wind) involved but buoyancy. Calculations use Archimedes' principle. The force comes from heat (combustion doesn't change the total mass of fuel+air, at least not in a significant quantity)

Any increase in buoyancy from the combustion products has got to be trivial.  I've never heard of water condensing on the inside of a hot-air balloon envelope and that certainly didn't happen on the handful of flights that I've been on.  But, I'll let someone else run through the math and get credit for a real answer.

Burning one mol of propane yields 3 mols of CO2 and 4 mols of H2O, so a total of 192g for 7 mols of gas.  Air has a mass of a little over 28g per mol so 7 mols equates to 196g.  So ignoring the temperature change the buoyancy is around 2% of the mass of gas.  That’s the equivalent of around a 2% change in absolute temperature, so heating the gas in the balloon by 6 or 7 degrees would have the equivalent effect.  A balloon can operate at up to about 120C, so the temperature differential is much more significant but the gas buoyancy does contribute.  It would make sense that water would condense on the fabric of the balloon, but a number of kg spread over the large surface would likely be absorbed by the fabric rather than raining on the occupants.
EDIT: as pointed out, the mass of combustion products is actually 204g so slightly denser than air.

You imply that 1 mol of reaction product replaces 1 mol of air. Avogadro's Law says we actually can make that assumption, at any fixed temperature.  The depletion of oxygen (which is partially - but a large part -  consumed in the flame) in the mix of hot air and combustion products will lighten things a little more, as N₂ is lighter than O₂.

I.e with about 4× as much N₂ as O₂ in air: C₃H₈ + 5O₂ + 20N₂  → 3CO₂ + 4H₂O +20N₂.  That's 25 mol (+1 from the gas tank) → 27 mol. With a mass of 720g of air + 44g of C₃H₈ gives 764g afterwards, but taking up 27/25 × the volume, so 707g in the same volume as the initial air - just over 98% rather than just under if you ignore the change in gas mix in air.  An interesting exercise for me, but a pointless one

@Chris H good point about the depletion of oxygen

It's also assumed here that the air in the balloon is completely replaced by combustion products. Since the air can mix freely with outside air I find this very unlikely.

@Chris H agreed.  However, combustion gases enter the balloon and force out cooler gas, so there will presumably be a tendency for air to be flushed out.  That said, to lift a payload of 100kg would require around 300kg of hot gas (assuming 270K external and 360K internal) so a 40kg LPG cylinder would only displace a fraction of the air.  Taking that into account the gas buoyancy becomes much less significant.

@Frog I like your answer but am unsure of your calculation for 7 moles of combustion products which I think is 3x CO2 (44g) + 4x H2O (18g) = 204g. This reinforces your argument so I conclude that the net effect of the combustion products is negative from the lift perspective.

@Ken Mercer you’re quite right, thanks for pointing that out

Pretty negligible. A typical balloon of volume $2800~\rm m^3$ at $100~\rm^\circ C$ at sea level pressure contains about $2600~\rm kg$ of air, or about 90000 moles.
If you burn $20~\rm kg$ (450 moles) of propane, this consumes 2250 moles of oxygen ($72~\rm kg$) and produces 1350 moles of carbon dioxide ($59.4~\rm kg$) and 1800 moles of water ($32.4~\rm kg$). Assuming all the combustion products stay in the envelope, this then displaces 900 moles of air ($26~\rm kg$).
So the new air mass inside the envelope is $$2600-72+59.4+32.4-26\approx 2594~\rm kg$$
So even with unrealistic assumptions where all the combustion products from about 10 gallons of propane all stay in the balloon, the density of air in the balloon is only decreased by about 0.2%. Assuming standard conditions, that means the $830~\rm kg$ being produced is increased to $836~\rm kg$, an increase of less than 1%.

the ballon rises as was commented by bouyancy. it simply displaces a volume of air that weighs more than it's own weight (cold air outside, vs the hot air inside), that's all there is to it.
water vapour in air is a complex subject, beyond avation, but water has a lower mol weight but it has a very strong hydrogen bond. so molecules will bond to each other. this is what rel% measures, how effective air molecules are in preventing that, or rather how much water vapour the air can carry before molecules start bonding and creating mist.
hot air is much drier (same ammount of water but lower rel%) so there's no condensation inside the balloon.
all other gases and subproducts are heavier molecules.

There is no lift being produced. The combustion of the fuel heats the air inside the balloon envelope and it expands, but the amount of air remains the same so the density of the air inside the envelope decreases. The surrounding colder air moves around and down the envelope. A convection current is created which forces the hotter air inside the envelope up. Since the air is trapped inside the balloon envelope, it too rises with the hot air.

I'm not the d'ver, but it almost sounds like you are saying that for a given net density of air within the envelope, and thus a given buoyancy, the balloon goes up faster if there's a convection current within the envelope that pushes up against the top of the envelope.

I did not say that, you did.

Ok.  But it's not clear how mentioning the convection current added to the answer.  PS -- re "but the amount of air remains the same"  -- amount meaning volume?  Certainly the number of molecules of air within the envelope does not remain the same when the air is heated.  Nor does the weight of the air.  Answer might be improved by clarification as to meaning of "amount".

Hot air balloons work because the air isn't trapped in them. When the air heats up, it pushes some of the air inside out of the mouth. The amount of air in the balloon goes down, but the volume stays the same, so the density goes down.

"the amount of air remains the same" Hot air balloons are neither rigid nor sealed, so I see no way that could be right.

Any air that escapes the envelope at the top will be replaced from the bottom. The volume of air that the balloon can hold at any given time remains pretty much constant. Physics 101.


text:
Never heard of this Aerofoil?
I just came across this aerofoil, 65-2xx?
I searched this but it doesn’t come up with anything like 65-200 for example but instead SG6051 which is the closest I could find to this?
Am I missing something here please?

You are not missing anything.  The notation, 65-2xx, is in reference to an NACA Series 6 airfoil having a lift coefficient of 0.2 and an undetermined thickness of 10 percent or less. As a comparison, the designation ${65_3}{-018}$ indicates an NACA Series 6 airfoil having minimum chordwise pressure at 50 percent of the airfoil chord, favorable minimum chordwise pressure existing at a range of lift coefficients 3 tenths above and below the airfoil design lift coefficient, a design lift coefficient of 0 (i.e., zero), and a design thickness of 18 percent of the airfoil chord.  Consequently, a ${65_3}{-218}$ airfoil would have a design lift coefficient of 0.2.
For clarification of the designation such as 65-2xx, the following is quoted for clarity from p. 6 of NACA Report No. 824, Summary of Airfoil Data -

For NACA 6-series airfoils having a thickness ratio less than 0.12 of the chord, the subscript number indicating the low-drag range should be less than unity. Rather than use a fractional number, a subscript of unity was originally employed for these airfoils. Since this usage is not consistent with the previous definition of a number indicating the low-drag range, the designations of airfoil sections having a thickness ratio less than 0.12 of the chord are now given without such a number. As an example, an NACA 6-series airfoil having a thickness ratio of 0.10 of the chord would be designated:  NACA 65-210

For the case at hand, 65-2xx is a generalized NACA designation for a similar group of these thinner airfoils.  Series 6 airfoils have been extensively used in applications for wings having low drag achieved through laminar flow.

That is an airfoil from the NACA 6-Series.
I quote the Wikipedia article linked above:

The number "6" indicates the series.
One digit describes the distance of the minimum pressure area in tenths of the chord.
The subscript (which is missing in your designation) digit gives the range of lift coefficient in tenths above and below the design lift coefficient in which favorable pressure gradients exist on both surfaces.
A hyphen.
One digit describes the design lift coefficient in tenths.
Two digits describe the maximum thickness as percent of chord.

That's why looking for 65-200 won't give you any results because that airfoil would have 0% thickness. 65-210, 65-212 or 65-215 would be "valid" airfoils.


text:
Reasoning for an inboard flap- take off
so Ive been looking into airplane flaps, like the longer one for take off-
Is it correct to say that the only reason to have one is for take off, to make it take off easier?
And so when they take off, they just put the flap fully down and take off quicker?

@JonathanChing - You seem to be very interested in this stuff.  The FAA have produced a book, available as a PDF; The Pilot's Handbook of Aeronautical Knowledge.  Chapter 6 is about flight controls such as the ones you are asking about - https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/phak/08_phak_ch6.pdf

Flaps are used for safe TakeOff and landing on fast aircraft. TakeOff without flaps could be possible, but a longer runway would be required to gather the extra speed. Landing without flaps means a prohibitively high landing speed.
Lift and drag on aeroplanes aree proportional to wing area S, wing profile curvature $C_L$, air density $\rho$, and airspeed squared, the well know lift and drag equations:
$$L = C_L \cdot \frac{1}{2} \rho V^2 \cdot S$$
$$D = C_D \cdot \frac{1}{2} \rho V^2 \cdot S$$
At high airspeed, the lift required can be produced by a relatively small wing area, which then also reduces drag so that less fuel is used for the trip. But with a relatively small wing area, flying slow can become an issue. To maintain enough lift to keep the plane up, the $C_L$ needs to be increased by flying with the nose up, and this has a limit beyond which the wing stalls and the plane will sink.

Flaps increase $ C_L$. The graph above was earlier used in this answer and shows how $C_L$ is increased at every angle of attack. At TakeOff flaps are used (partly deployed) on faster planes, but they are particularly important durimg landing when they are deployed fully. Fast jets often have Fowler flaps, which increase both the $C_L$ and the wing area.

Flaps aren't just for fast aircraft.  I used to fly a slow, heavy, two-seater spam-can that cruised at 80kn and I always used flaps to take off (and for landing, obviously).  Without take off flaps, it could take a week or two of lumbering down the runway before I could coax it into the air :)

It is correct to say it makes take off "quicker", and in many cases it is that it makes take off "possible", at least for larger airliners - a light aircraft can often make a safe take off with no flaps extended.
The problem with large, heavy airliners is that the stall speed with no flaps is higher than the speed that can be attained on a given length of runway. In order to take off with more weight, or with a shorter runway, that stall speed needs to be reduced, which is done by increasing the lift produced by the wings, which is done by increasing their surface area and coefficient of lift, which is done by extending the flaps.


text:
How are NATs connected to European Airways?
The North Atlantic Tracks connect Northern Europe to North America. To enter/exit NATs over North America, the North American Routes (NARs) are used.

Questions:
What about the European side? What are the rules to enter/exit NATs? What airways must be flown? Can I find the official information written anywhere?
Note: Ideally I would like to find a list of routes (directs?) that suggests the pair of waypoints linking the generic NAT and the european airways system. In the past North Atlantic European Routing Scheme (NERS) was used but nowadays it appears they have been dismissed.

Partly answered (see this map for instance) in: Where to download NATs (North Atlantic Tracks) information?

@mins the question is not duplicate since my question asks for information about the part of airspace connecting the NATs and the european airwways. I am not asking for NATs information where to download it.

There is no equivalent in Europe to the North Atlantic Routes, you simply file direct from point to point.
This is the routing for a flight on Wednesday 18th of January, 2017 (DAL4).
KJFK
MERIT HFD PUT WITCH ALLEX N263A JOOPY NATS RESNO NATS NETKI NIBOG REMSI UL603 SOGPO UT149 OTSOP T149 LIPMI T150 ROLIS
EDDF

You see that the route terminates NAT S at NETKI NIBOG REMSI which are directs, until the flight joins the upper airway UL603 at REMSI.
Here is the routing in SkyVector.
And here is the screenshot from the FlightAware flight.

(Image Source: FlightAware)

For some reason SkyVector doesnt like NIBOG.

Thanks for the answer. Flying directs is something I was expecting. However I believe there must be a list of directs that suggests the pair of waypoints linking the generic NAT and the european airways system no?

The flights on the European side are validated through CFMU for routing and slot management, but essentially, you will not have a list but just need to pass the route through CFMU. If CFMU doesnt object due to restrictions, then you are free to file it. AFAIK.

What is CFMU and AFAIK? I also suppose that only certain waypoints from the european side may be selected for directs to NATs. Otherwise any waypoint would be acceptable no?

CFMU and AFAIK --> As Far As I Know

I also suppose that only certain waypoints from the european side may be selected for directs to NATs. Otherwise any waypoint would be acceptable no?


text:
Why is UTC activation time referred to 30W in NAT-OTS?
I am reading the NORTH ATLANTIC OPERATIONS AND AIRSPACE MANUAL and I came across the following statement more than once during my reading:

The activation time of the westbound OTS shall be published as 1130z
  to 1900z at 30W.
The activation time of the eastbound OTS shall be published as 0100z to
  0800z at 30W.

You can find this quotes at page 175 of the link provided above.
Here is my question: if the activation times are expressed as ZULU time (therefore as UTC), why is it specified at 30W? As far as I know a ZULU time is referred to the Greenwich meridian and therefore at 0W. To me it sounds quite contradictory to express an UTC time by specifying a longitude different than 0.
What am I misinterpreting? What is the right interpretation of the quotes presented above?

yes sorry about that

does this mean that if I cross for example 30W at 11.29 I will not be flying on a westbound track for the rest of the flight (according to the flight plan)?

Think of 30W as a kind of traffic marker for flight planning.
If your planned route is going to cross 30W before the tracks activate (that is, ahead of the traffic flow), then you can file "random route." If you're crossing 30W during the active period, then you have to file on one of the tracks unless you can climb above them before crossing the entry fix.
This also works going against the flow: if you're crossing 30W in the opposite direction during the active period, you'll have to route either north or south of the tracks unless you can get above the NAT FL's before your entry fix.
As far as Zulu goes, it's just the common reference everyone uses (therefore, "Universal Coordinated Time").

Overall facts behind the NAT OTS:

There are strong winds in altitude over the Atlantic Ocean (jet streams), including the northern polar jet stream over the North Atlantic Ocean, which is blowing from west to east.
Airlines try to take benefit of these tailwinds when flying from NA to Europe, and they try to find routes with the weakest headwind velocity when flying from Europe to NA.
The result is that at peak times, which are different for each direction, there are concentrations of aircraft at the same location.
To maximize traffic, airlines are using a collaborative system, the NAT-OTS where routes are optimized each day for winds and peak times, in each direction.
Due to passenger preferences for traveling, peaks happen at known times, and at 30W longitude (the middle of the Pound), therefore the estimated time for a flight crossing 30W meridian is a first importance.
The OTS activation periods you mentioned are these peak periods, expressed in UTC, for the two traffic directions when aircraft are crowded together over 30W locations.
Shanwick and Gander Oceanic Centers have a common border at 30W, as visible here:

From the elements above, "The activation time of the westbound OTS shall be published as 1130z to 1900z at 30W" can be understood this way:

The westbound OTS is activated for the westbound peak traffic over 30W longitude.
The westbound tracks are active (in this example of track message) between 11:30 UTC and until 19:00 UTC.
These tracks are optimized furthermore to deal with the spatial peak occurring at 30W longitude.